This invention relates to electronic musical instruments, and more particularly to an electronic musical instrument of a type wherein musical tones are produced by executing in a digital mode the calculation of an equation representing frequency modulation signals.
An electronic musical instrument of this type is based on a principle that musical tones are produced by calculating the following Equation (1) known as an equation representing frequency modulation signals, and is disclosed by, for instance, U.S. Pat. No. 4,018,121. EQU e=A.multidot.sin (.omega..sub.c t+I(t).multidot.sin .omega..sub.m t) (1)
The spectrum distribution of musical tones produced by the conventional electronic musical instrument of this system is determined by relationships between the angular velocity .omega..sub.c of the carrier waver term .omega..sub.c t and the angular velocity .omega..sub.m of the modulation wave term sin .omega..sub.m t of Equation (1). However, depending on the relationships between the two angular velocities .omega..sub.c and .omega..sub.m, there may happen a case wherein all the frequency components necessary for a certain tone color of a musical tone to be produced are not included. Accordingly, the tone color of a musical tone which can be produced by the conventional electronic musical instrument is limited.
The levels of the frequency components of the musical tone thus produced are, in general, determined by the selection of the value I(t) in Equation (1). In this connection, if one of the frequency components is selected to a suitable value, the values of the remaining frequency components become necessarily undesirable ones; thatt is, it is very difficult to set the levels of all the frequency components of a musical tone to be formed (which is, in general, made to be similar to one produced by a natural musical instrument) to optimum values.
For instance, natural musical instruments are characteristically different from one another in frequency spectrum aand frequency component level in accordance with the tone colors thereof. For instance, the piano includes both of harmonic tones and non-harmonic tones in its spectrum. Furthermore, natural musical instruments of a reed type include only frequency components of odd-number harmonics.
Accordingly, in order to form musical tones which are similar to those formed by the natural musical instruments, it is necessary to obtain spectrum distribution satisfying the characteristics of various tone colors and to be able to set the levels of various frequency components included in the musical tones to specific values.
However, heretofore it is considerably difficult for some of the conventional electronic musical instruments to simultaneously effect the selection of the angular velocities .omega..sub.c and .omega..sub.m and the selection of the value I(t).
On the other hand, in the conventional electronic musical instrument, as shown in FIG. 17, the pitch of a musical tone to be formed is specified by applying the output of a key switch circuit KS to a musical tone signal forming circuit TGR, while parameters set and stored in advance are successively read to be applied to the musical tone signal forming circuit TGR by means of a parameter circuit PRM, and the calculation of the fundamental equation is carried out according to these pieces of information in the musical tone signal forming circuit TGR, the calculation result being applied, as a musical tone signal, to a sound system SUD.
However, such a digital type electronic musical instrument cannot give a desired tone color and a desired envelope to a musical tone produced by depressing a key or keys. Accordingly, musical tones formed by the conventional digital type electronic musical instrument is much different from those formed by the natural musical instruments.
In other words, a musical tone produced by a natural musical instrument has variations in tone color and envelope depending on the tone ranges. For instance, musical tones produced by the piano are short in envelope and small in tone volume in the high tone (treble) range, but are long in envelope and large in tone volume in the low tone (bass) range. Furthermore, in the bass region, the musical tones produced by the piano have a large percentage of partial tone contents and have a greater inharmonicity.
Accordingly, in the case where it is intended to form piano tones with the electronic musical instrument, it is effective for the purpose of making musical tones formed by the electronic musical instrument similar to those formed by the natural musical instrument as much as possible that the electronic musical instrument is so designed that the tone colors and envelopes of musical tones produced thereby can be varied according to the tone ranges assigned to the keys or according to the key positions so as to conform the characteristics of the musical tones formed by the natural musical instrument.